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Join Date: Feb 2014
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# 4KW Power Station Problem

03/21/2014 5:24 AM

Gents,

But Now I just ask for an active solution for this problem.

Our power station consists of four generators 1MW each A problem occurred to Gen#2 causing Rewinding of its main stator , When we put it in synchronizing to power station a very high current drawn by neutral conductor around 1/4 of generator O/P current the neutral to earth voltage of gen#2 is around 3.4v .

We guess the main cause of the problem is due to different in voltage shape produced by gensets after Gen#2 rewinding but we cave earth fault protection by CT located on neutral conductor caused tripping of gen#2 we calibrated it @ 0.35 of nominal generator current.

Our loads are basically electrical Submersible pumps (ESP) fed from VSD which cause large quantity of harmonic.

Every VSD has its own harmonic filter but when we measure harmonics on PCC (common B.B) feeding the loads the reading was:

Voltage (THD) = 8%

Current (THD) = 35%

Which exceed harmonic limits defined by IEEE-519 Std. and all of harmonic caused by 5th harmonic.

1. What are the main cause of 5th harmonic and there effect on power station?

2. I know that high neutral caused by 3rd harmonic but the value of 5th is much higher from 3rd (I couldn't understand).

3. The direction of current flow through gensets is out from Gen#1, 3, 4 entering Gen#2, so why gen#2 is calling for high neutral current and why this not happened with other gensets.

4. We have 2 Accusine PCS active harmonic filters works in parallel with 50A (corrective cuurent each capacity) every time we try to put it in service complete black out occurred the protection modules states different faults the most common are loss of excitation and reverse reactive power although we used it as harmonic filter not for PF correction (how can we avoid?)

Any further advice would be appreciated.

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#1

### Re: 4KW Power station problem

03/21/2014 7:15 AM

As indicated before, please confirm that only 1 genset is ever connected to neutral.

As the topic has been raised before, does this qualify as a duplicate thread under CR4 posting rules?

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#2

### Re: 4KW Power station problem

03/21/2014 8:04 AM

my last thread was only about high harmonic current but now i ask about harmonics and its effect and how can we eliminate.

And ur idea about connectting only generator neutral is really good but i try the definite solution of this problem.

Thx PW for ur concern .

Guru

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#3

### Re: 4KW Power Station Problem

03/21/2014 12:09 PM

i'll only address question 3, but somehow I think all the other answers are dependent on what exactly happened to generator 2. There are facts that are needed for a thorough analysis:

1. Are all the generators exactly the same?

2. What exactly happened to gen2 that required a complete rewind?

3. Was the rewinding of gen2 witnessed and the winding data recorded to ensure that it is wound exactly like the other three?

4. While gen2 was out of service was any of the ancillary equipment like the excitation system, AVR, PTs, CTs, etc., checked, replaced, refurbished, recalibrated?

5. How are the AVRs and governors set; isolated, synchronized, master, slave, etc.?

6. Is there any form of cross-current compensation, VAR-sharing, droop, load-sharing, that is properly setup?

7. Why do you continue to operate gen2 when it is absorbing all that VAR loading?

There are a lot of (to be kind) unusual operating conditions/warnings that continue to be ignored, perhaps the answers to the above questions will assist in regaining control of this rudderless ship.

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Guru

Join Date: Apr 2008
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#4

### Re: 4KW Power station problem

03/21/2014 12:16 PM

Perhaps your problems more complicated than you think. Power factor correction capacitors will also interact as part of a large RLC circuit creating a reasonant condition that has a fundamental frequency of your 5th harmonic.

Take a look at this link for white papers on power quality.

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#5

### Re: 4KW Power Station Problem

03/21/2014 3:39 PM

1. Are all the generators exactly the same?

Yes, All the gensets are identical , All 1MW capacity with brand CAT-3512B

2. What exactly happened to gen2 that required a complete rewind?

Gen#2, Insulation failure of main stator causing high S.C current Which burnt the main stator and cause serious damage to main rotor

3. Was the rewinding of gen2 witnessed and the winding data recorded to ensure that it is wound exactly like the other three?

Yes, But I couldn't confirm they are typical 100% We have received the genset and tested it with rewound CO. Rep. and he confirmed that They rewound genset WRT old rewound.

4. While gen2 was out of service was any of the ancillary equipment like the excitation system, AVR, PTs, CTs, etc., checked, replaced, refurbished, recalibrated?

No, We just isolate alternator and send it for rewinding.

5. How are the AVRs and governors set; isolated, synchronized, master, slave, etc.?

Well, Governer is controlled by woodward LSM and AVR are CDVR and Synchronizing modules are deepsea which control synchronizing and active,reactive power sharing.

6. Is there any form of cross-current compensation, VAR-sharing, droop, load-sharing, that is properly setup?

Yes, All the control connections still like it was before rewinding Gen#2, and after we receive generator we return back connections like they are .

7. Why do you continue to operate gen2 when it is absorbing all that VAR loading?

Well, You have misunderstand this point , All generators are sharing active and reactive power properly but only generator#2 call for very large current on neutral

There are a lot of (to be kind) unusual operating conditions/warnings that continue to be ignored, perhaps the answers to the above questions will assist in regaining control of this rudderless ship.

3
Guru

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#6

### Re: 4KW Power Station Problem

03/21/2014 5:05 PM

Generally free advice from strangers who do not even know the brand/model/wiring scheme/ etc. etc. etc. etc. of the generators you want to cure will never result in a satisfactory resolution.

Contact the customer service department of the gen set maker and ask them for help. They should send a field tech to do this, as your staff doesn't seem to have the required skills.

Of course, unless your generators are new, they will require payment for their time.

The responding members here (me excepted) are all quite knowledgeable in their fields but can't read minds or work miracles.

Seek competent help, now.

Good luck!

Guru

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#7

### Re: 4KW Power Station Problem

03/21/2014 5:25 PM

Your title states "4KW Power Station Problem"

In the body you refer to: "four generators 1MW each"

Is it 4KW or 4MW? That is a factor of 1,000 either way.

Good Luck, Old Salt

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#8

### Re: 4KW Power Station Problem

03/22/2014 12:27 AM

Since the problem occurred after the rewind, and it only occurs with this generator, I suspect a winding problem on the rewind, or if any devices are only in service at the time of adding this generator in parallel, check them also. I always go to the last change made before a problem occurs. That is usually it. coincidences happen, but rarely.

Commentator

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#9

### Re: 4KW Power Station Problem

03/22/2014 2:14 AM

It could be inferred well that the issue is with gen #2 as issues cropped up after it's rewound ... but I feel - check the system without G#2 and - G#2 individually - for proper functioning. ensuring the function of two packets are in order. Secondly - the issue is it is drawing high neutral current ... Check parameters of windings and earth / star lead resistance to phase windings ... whether there is any leakage to ground >>> If rest of the system functions well, detailing on G#2 is the only solution... as suggested differently by othere s here. IF HARMONICS ARE ONLY CAUSE THEN THE UNIT WILL ALSO HEAT HEAVILY I9 SUPPOSE.

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#10

### Re: 4KW Power Station Problem

03/22/2014 7:08 AM

You have two different problems.

Ist Circulating currents on synchronizing of rewound Alternator.

2nd Tripping of harmonic filters on switching.

Problem to 1st appears in polarity and location of Droop CT.

Generally for most of alternator manufacturer, practice is to locate Droop CT in middle phase (phase 'V'). Pl check if this is correctly installed.

Next is whether it is threaded correctly by the Re-winder, that is polarity is correct.

If above solves the problem let us know.

2nd issue is tripping of Filters on switching on. This can be due to inrush current of capacitors (part of LC circuit). This may be happening sometime. 2 possible solutions are:

1. Install resistance in series (say 10 ohm Volt square/10 watt) with filter and 3 pole 50A AC1 duty power contactor to shunt it after few seconds.

2. Or Connect the filter to bus and one of 3 DG sets to bus before starting of 1st DG set. The charging current will build up slowly as the voltage of DG sets /Bus builds up.

Only negative point which disfavors alternative 2 above is that capacitive currents are not good for Alternator - hence some load must be switched on as soon as possible after start of fist DG.

Other question asked was effect of harmonics:

1. Distorts your voltage wave form and thus even can be problem for sensing circuits of UPS and VFDs.

2. Wrong sensing of output voltage of generators by AVR and thus erratic behaviour of AVR. The AVR is supposed to sense Fundamental component. Sometime we have to install a low pass filter in sensing circuit.

3. Overheating of alternator due to increased eddy and hysteresis losses.

We shall maintain distortion at terminals of alternator to 7% and never exceeding 9%. This can be calculated from value of individual harmonics and sub-transient reactance (Direct Axis) of the machine. Lower Xd", lesser is distortion at terminals of alternator.

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#11

### Re: 4KW Power Station Problem

03/22/2014 11:51 AM

Thanks PowersolutionFBD for ur concern

Generally for most of alternator manufacturer, practice is to locate Droop CT in middle phase (phase 'V'). Pl check if this is correctly installed.

I agree with you but I can get only phase rotation couldn't get that this phase is U or V or Z (Could you tell me to know the phases)

Install resistance in series (say 10 ohm Volt square/10 watt) with filter and 3 pole 50A AC1 duty power contactor to shunt it after few seconds.

This is already existed in harmonic reduction (Instruction bulletin 5820IB0801 Rev 1)

Connect the filter to bus and one of 3 DG sets to bus before starting of 1st DG set. The charging current will build up slowly as the voltage of DG sets /Bus builds up.

This is very difficult to be done through we have to do complete S/D to power house with a very high cost of loss of production

THX

Guru

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#12

### Re: 4KW Power Station Problem

03/24/2014 10:42 AM

Have you identified the original root-cause of the Gen 2 failure?

I suspect that the issues you are now experiencing are related to the failure of Gen 2 especially given that all four generators are in parallel.

I would start with a close in-depth investigation of the system and individual controls because a control or filter component failure could cause the generator winding to fail and vice-versa wherein the generator winding failure could have caued control and/or filter component failure.

I would suggest that you aquire or rent a "dummy" load set of the appropriate KW size to assimulate the generator output then disconnect and test the Gen2 separate from the system to identify critical output voltage level and waveform shape. (Much cheaper than a rewind.)

If it were me, I would contact and have the factory field service expert(s) on site to direct all testing.

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#13

### Re: 4KW Power Station Problem

03/24/2014 11:22 AM

Generally for most of alternator manufacturer, practice is to locate Droop CT in middle phase (phase 'V'). Pl check if this is correctly installed.

I agree with you but I can get only phase rotation couldn't get that this phase is U or V or Z (Could you tell me to know the phases)

It is very simple. You have 3 machines, out of which 2 are healthy. Take one healthy machine say DG1 and the faulty machine, DG2. Charge bus with healthy machine. Take 2 AC voltmeters of range double of machine voltage. Connect one meter between "R" of DG 1 and "R assumed" of DG 2. Similarly connect "Y" of DG1 and "Y" assume of DG2" . If the assumed polarity of DG2 is correct then both voltmeters will simultaneously read minimum and maximum values.

If not, then, check "Y" of DG1 with "Y assumed" of DG2. In this case above observation must be observed. Where you find above happening that will be the "Y" phase of faulty macine.

Install resistance in series (say 10 ohm Volt square/10 watt) with filter and 3 pole 50A AC1 duty power contactor to shunt it after few seconds.

This is already existed in harmonic reduction (Instruction bulletin 5820IB0801 Rev 1)

If you have a resistance in series then check your protection.

Connect the filter to bus and one of 3 DG sets to bus before starting of 1st DG set. The charging current will build up slowly as the voltage of DG sets /Bus builds up.

This is very difficult to be done through we have to do complete S/D to power house with a very high cost of loss of production

THX

This is not difficult - needs proper planning. If you can not do, better hire someone.

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#14

### Re: 4KW Power Station Problem

03/30/2014 1:00 AM

IMO the fault generator its self is the source of 5th harmonics. The stator winding could be a fractional-slot winding-that means the number of slots per pole and phase [q=Nslots/(2p)/3] fractional .

For instance from "General Characteristic of Fractional Slot Double Layer Concentrated Winding Synchronous Machine" by Un-Jae Seo*, Yon-Do Chun†, Jae-Hak Choi*, Shi-Uk Chung* and other.

"Fractional-slot concentrated winding (FSCW) machines are very attractive solution in several applications. These advantages include high efficiency, short end turn, high slot fill factor especially used with segmented stator core. The drawbacks of fraction-slot configurations are a slightly lower winding factor and a high harmonic content in the MMF distribution compared to the integral slot machine.

The undesirable low order vibrations, local saturations are occurred due to these harmonic contents."

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#15

### Re: 4KW Power Station Problem

04/05/2014 12:37 AM

I apologise in advance if this comment came through a few time, there seems to be a problem with my PC...

If the star point of all generators are connected together, I would suspect the possibility of the pitch of the rewound generator is not quite the same as the others. It might be worth getting a power analyser to check the harmonics in the voltage for all machines. If the new one is significantly different, then that could be your problem.

For more information, see below link for a basic paper about machines with different winding pitches and if inclined, suggestions on how to minimise the circulating currents when operating in parallel.

https://www.cumminsgeneratortechnologies.com/www/en/media/whitepapers/Winding%20Pitch%20White%20Paper_EN_LR.pdf

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